One type of the above specified methods is based on phase interferometry and moire principles in accordance with which an optical object is inspected by the detection and analysis of interference and moire patterns created by the interaction of inspection radiation which passes through the optical object with reference radiation or a grating. Examples of methods of this type are disclosed, for example, in U.S. Pat. No. 4,459,027 and U.S. Pat. No. 4,810,895. However, these methods suffer from low resolution and require complicated interpretation of the produced images, which renders these methods inefficient, slow and expensive.
U.S. Pat. No. 5,309,222 and DE 3 237 511 disclose methods of another type, in which optical objects to be inspected are placed on an optical path of a television camera and are used to display a test pattern on the camera. A video signal obtained thereby is compared with a control signal not influenced by the inspected object and, based on a deviation therebetween, flaws in the inspected object are identified. However, methods of this type are suitable only for the inspection of flat objects. In addition, they are not capable of the detection of small flaws.
There also exist well known "shlieren" photography and "shadowgraphy" inspection methods which are based on the analysis of a shadow picture obtained as a result of phase changes in inspection radiation beams passing through or reflected from inspected optical objects. However, since in such an analysis the shadow picture depends on the optical power of inspected objects, it is extremely difficult and often almost impossible to use this analysis in automated quality control where pictures obtained from different inspected objects must be captured by one pre-set imaging device. In addition, the known methods of analysis of phase objects cannot determine accurately actual dimensions and locations of the object's flaws.